1. Field of the Invention
The invention relates to improved race elements in a rolling element bearing assembly. It is particularly directed to an improvement in the composition of the race elements accomplished by functionally gradating the race material composition to optimize corrosion resistance, thermal expansion, hardness of the raceway, and minimum friction at the rolling element to raceway interface.
2. Related Art
Most prior art is for rolling element bearings operating in oil or a liquid that provides lubrication as well as cooling. For dry operation or for machinery where the rolling element bearing must operate in a fluid with low viscuous properties such as cryogenic propellants, conventional bearing materials cannot be utilized. Corrosion resistant materials, such as martensitic stainless steel, are presently used for the races and Teflon (Trademark of E. I. duPont de Nemours & Co., Inc.) filled fiberglass or silver are used for the dry lubricating cage or separator.
Recent improvements in rolling element bearings have included silicon nitride balls or rollers which are much harder and more wear resistant than martensitic stainless steel. When silicon nitride balls or rollers are operated with martensitic stainless steel races, the raceways are the first to wear and spall. In addition, the martensitic race material thermal coefficient of expansion is less than that of the typical shaft and housing materials. The inner race must, therefore, be installed on the shaft with an interference fit at room temperature in order to remain piloted at the cryogenic operating temperature. This results in high hoop stresses in the martensitic stainless steel inner race which is susceptible to stress corrosion when in the presence of moisture, resulting in inner race cracking. Conversely, the outer race must be installed in the housing with a larger than desirable clearance to compensate for the difference in thermal contraction.
Various bearing assembly configurations each provided with lubrication means are disclosed such as for example in the following documents incorporated by reference.
U.S. Pat. No. 4,362,344 of Dec. 7, 1982, provides a bearing with stamped, or ground, inner and outer races in which the gap between the races is filled with a separate rubber sealing ring. In addition a full complement of bearing elements along with the desired type and amount of lubricant is assembled between curvilinear raceways for controlling bearing assembly motion.
U.S. Pat. No. 4,293,171 of Oct. 6, 1981 describes an anti-friction bearing wherein the rolling elements are coated with an intermediate metal layer such as by ion plating, said metal layer having lubricating properties.
However, it is heretofore unknown to provide, in a bearing assembly, race elements which are functionally gradated to provide corrosion resistance, desired thermal expansion, hard raceway wear surface properties, and minimum friction.